A 13-year-old boy who was one in only five people in the world with an extremely rare cancer that meant he had only two weeks left to live managed to make a miracle recovery – thanks to cannabis.

Deryn Blackwell was diagnosed with leukaemia when he was just ten years old in 2010.

Just over a year later he was told he had a secondary cancer – Langerhans cell sarcoma -, which only five people in the world currently have.

Yet no one had ever been found to have the two cancers together making Deryn one in seven billion people to have it.

After nearly four years of hospital treatment, nothing was working – so his mother Callie resorted to trying to give Deryn Bedrocan, a cannabis-based painkiller that wasn’t available in the UK.

But despite doctors saying it could be effective, it had not been tested on children so she couldn’t prescribe it.

In an except from hew new book The Boy In 7 Billion, published in the Mail on Sunday, Callie said: ‘We took a decision that will horrify many parents reading this – and horrified me, too.

‘After all, I’d never seen anything positive come of smoking cannabis, and in my days working in nightclubs, illegal drugs had been my enemy.

‘But if it could help my darling boy escape his daily torment, I was willing to try it.’

They then set about finding some cannabis – Deryn’s dad, Simon, arranged to meet someone at a nearby petrol station to pick some up – despite them knowing it was a class B drug, which carries a sentence of up to five years’ imprisonment for possession.